Millimeter Wave Spectrum Analyzer with Built-in >100 GHz Preselector
Yukiyasu Kimura, Masaaki Fuse, Akihito Otani
[Summary] Fifth-generation (5G) mobile communications technologies are being actively developed worldwide
and measuring instruments supporting frequency bands above 100 GHz are urgently needed.
However, there is as yet no tunable filter for use as a preselector at spectrum analysis in the re-
quency bands above 100 GHz. To meet this need, we have developed a mechanically tuned filter
using a Fabry-Perot resonator incorporated into a waveguide for use as a preselector. Additionally,
we have greatly reduced distortion to generate a high-purity local signal, supporting more accurate
measurement by using a millimeter wave generation method using an optical 2-tone signal pro-
duced by the coherent interference method at generation of a local signal using a block down con-
verter. This has been used to develop a millimeter wave spectrum analyzer supporting frequencies
above 100 GHz. Using this system, we have successfully measured a 120-GHz, 20-Gbps band
QPSK modulation spectrum, verifying that the developed preselector suppress spurious response
and assures accurate measurement.
(1)
1 Introduction
Research and development of fifth-generation mobile
communications methods (5G) targeting 1000 times the
capacity of present-day LTE methods is pressing ahead in
recent years. Development of the millimeter band is making
rapid progress as a frequency band supporting delays of less
than 1 ms at data rates about 10 Gbps, and there have been
positive results in implementing wireless systems at speeds
exceeding 100 GHz. However, there is no spectrum analyzer
with a built-in preselector for spectrum analysis of wireless
signals exceeding 67 GHz. Consequently, these measure-
ments are generally made by connecting either an external
mixer or a block down converter1), 2). In these cases, signal
components that don’t exist in the original input signal are
observed. As a result, the functions of the spectrum ana-
lyzer are inadequate for monitoring unknown signals. To
solve this problem, we have developed a millimeter-wave
with newly fabricated built-in preselector supporting fre-
quencies above 100 GHz. We have demonstrated the ability
of this specyrum analyzer to measure the spectrum of
20-Gbps band QPSK modulation wave at 120 GHz as well
as suppression of spurious response using the preselector.
2 Millimeter Wave Spectrum Analyzer
This spectrum analyzer supports signals from 110 GHz to
140 GHz.
Figure 1 shows the external appearance and Figure 2
shows the block diagram. As seen in Figure 1, the system is
composed of a Head with WR-08 waveguide input, a com-
mercially available spectrum analyzer, (SA) and an optical
local signal source (Optical LO Signal Generator). As seen
in Figure 2, the Head is composed of a tunable Preselector,
directional coupler (Isolator), reference wave mixer (Mixer),
amplifier for tuning the level (Amplifier), and UTC-PD
(Uni-Traveling-Carrier Photodiode)3) for converting the op-
tical local signal to an electrical signal. Using the
down-conversion method, the mixer creates multiple mixing
products in the output IF signal determined by the mixer
LO frequency fLO and the RF signal fRF. The RF frequency is
obtained as both local frequency ± IF4). As a result, a con-
ventional SA is constructed using a preselector to suppress
the sensitivity to unwanted frequency components. How-
ever, unlike the YIG Tuned Filter (YTF) used as a preselec-
tor in the microwave band, there is still no tunable filter
available for frequency bands above 100 GHz. We have de-
veloped a mechanically tuned Fabry-Perot resonator5) fab-
ricated in a waveguide as the preselector for the Head used
with this SA.
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Anritsu Technical Review No.23 September 2015 Millimeter Wave Spectrum Analyzer with Built-in >100 GHz Preselector
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Figure 1 Millimeter Wave Spectrum Analyzer
Figure 2 Millimeter Wave Spectrum Analyzer Block Diagram
2.1 Preselector
The developed preselector controls the oscillator length L
to function as a millimeter-wave tunable filter with variable
tuning frequency fTUNE.
Figure 3 shows an external view of the preselector and
the principle of operation. As seen in Figure 3, the prese-
lector is composed mainly of a filter composed of waveguides
and an actuator for driving the filter. The actuator drives
one side of the half mirrors forming the Fabry-Perot reso-
nator inside the waveguides to control the tuning frequency
fTUNE by controlling the oscillator length L.
Figure 4 shows the frequency characteristics of this pre-
selector; Figure 4a shows the S21 transmission character-
istic when the oscillator length L is 1.3 mm. Figure 4b
shows a magnified view around the center frequency; 3 dB
down from the peak level at about 124.9 GHz, the band-
width is about 400 MHz and the attenuation at 5 GHz from
the center frequency is about 30 dB. Figure 4c shows a plot
of the maximum value of the S21 transmission characteris-
tic vs tuning frequency fTUNE. The degraded noise perfor-
mance due to the preselector is less than 6 dB in the fre-
quency band between 110 GHz and 140 GHz. Figure 4d
shows the frequency setting deviation vs tuning frequency
fTUNE. It is less than 0.1 GHz in the frequency band between
110 GHz and 140 GHz.
2.2 Synchronous Operation Principle
To analyze 110 GHz to 140 GHz RF signals, this system
uses a method incorporating a preselector for suppressing
generation of image signals into a block down converter.
Conventionally, a sweep-type SA with built-in preselector
uses method for synchronizing the mixer local frequency
and preselector4). However, when sweeping the local fre-
quency at frequencies above 100 GHz, complications such as
the generation method and filter structure be expected. As a
result, in this system, the preselector tuning frequency uses
a step operation and the SA frequency range of 110 GHz to
140 GHz is division-swept. The preselector tuning frequency
fTUNE and SA analysis center frequency are tuned while
separately sweeping the range of 110 GHz to 140 GHz to
finally generate the measurement band spectrum. When
using this type of divided sweeping method, the level re-
producibility and sweep speed can be expected to be affected
by the preselector tuning frequency fTUNE step interval. In
this system, the tuning frequency step gap was set to 0.1
GHz based on the based on the setting reproducibility of the
used preselector (Figure 4d) and the passband width. When
measuring at a frequency range (SPAN) of 30 GHz and res-
olution bandwidth (RBW) of 1 MHz, the sweep speed is
about 15 seconds.
Figure 3 Preselector
L
Half mirrors
waveguide
waveguide
Fabry-Perot resonator
(a) External View (b) Operation Principle
Filter
Actuator
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Anritsu Technical Review No.23 September 2015 Millimeter Wave Spectrum Analyzer with Built-in >100 GHz Preselector
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Figure 4 Preselector Frequency Characteristics
2.3 Optical Local Signal Source
This system generates the local signal using a millimeter
wave generation method using coherent interference7), 8).
Figure 5 shows the block diagram of the optical local
signal source. The optical local signal source is generated as
an optical 2-tone signal (Optical LO Signal) with a fre-
quency error of four times the signal frequency fm generated
by the signal generator; generated optical 2-tone signal is
converted to an electrical local signal corresponding to the
frequency error (4•fm) by the UTC-PD in the head for use as
the reference wave mixer local signal. Using this method
makes it possible to generate a high-frequency millimeter
wave signal easily and reduces the size of the Head size.
Figure 5 Optical Local Signal Source Block Diagram
Figure 6 shows a maximum values for low-frequency dis-
tortion (1 ×fm, 2 ×fm, •••n ×fm) due to coherent interference
millimeter wave signal generation, and passive electrical ×4
and active electrical ×4 multipliers. When using an electri-
cal multiplier, the maximum generated low-frequency dis-
tortion is about −3 dBc, but with the coherent interference
millimeter wave generation method, it is only about −60 dBc.
Assuming that the attenuation of the general filter is on the
order of 40 dB, using the coherent interference millimeter
wave generation method for the frequencies used by this SA
(fLO = 106.4 GHz), produces a much higher purity signal
using a filter or more than one stage to eliminate spurious
in comparison to the electrical multiplier method. Filter loss
makes a big contribution to reducing the size of the detector
built into the preselector and cutting power consumption.
Figure 6 Comparison of Optical Local Signal Source Harmonic
Distortion Level Results
3 Measurement Results
To confirm the effectiveness of the preselector in this
system, we measured both the spurious of a CW signal, and
the spectrum of a wideband modulation signal.
3.1 Spurious Measurement using CW Signal
A CW signal in the measurement band (110 GHz to 140
GHz) was input to the system and the observed spurious
response level was measured.
Figure 7 shows the block diagram of the system for
measuring spurious using a CW signal. The signal fre-
quency from the signal generator (SG) is multiplied by 8
using the Multiplier and this frequency multiplied signal is
passed though a bandpass filter (BPF), and variable atten-
uator (VATT) before input to the measurement system. The
input level Pin at the Head input connector was adjusted to
–10 dBm using the VATT to measure the spectrum at this
time; analysis of the spectrum results was used to give the
input level vs spurious level. Moreover, to suppress the
spurious level in the measured signal source, measurement
was performed while changing the BPF according to meas-
ured frequency.
LN
modulator
Driver
Amplifier
Signal
Generator
Fiber
fmf
Optical LO Signalλ
4·fm
LD BRF EDFATo UTC-PD
in Head
fm = 26.6GHz
( fLO = 106.4GHz )
100 101 102 103 104 105 106 107 108 109 110-100
-80
-60
-40
-20
0
Frequency [GHz]
Le
ve
l [d
Bc]
Optical x4
Electrical (Active) x4
Electrical (Passive) x4
110 120 130 140−10.0
−8.0
−6.0
−4.0
−2.0
0.0
Frequency fTUNE
[GHz]
Maxim
um
valu
e o
f S
21 [
dB
]
110 120 130 1400
0.02
0.04
0.06
0.08
0.1
Frequency fTUNE
[GHz]Vari
an
ce
of
Ce
nte
r F
requ
ency σ
f C [G
Hz]
(c) Maximum value of S21 (d) Frequency Setting Deviation
(a) S21 (Full spectrum) (b) S21 (Magnified)
110 120 130 140−40.0
−30.0
−20.0
−10.0
0.0
Frequency[GHz]
S21 [
dB
]
L = 1.30[mm]
124.5 124.6 124.7 124.8 124.9 125 125.1 125.2 125.3−8.0
−7.0
−6.0
−5.0
−4.0
−3.0
−2.0
Frequency[GHz]S
21
[dB
]
L = 1.30[mm]
3 dB
400 MHz
Frequency [GHz] S
21
[d
B]
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Anritsu Technical Review No.23 September 2015 Millimeter Wave Spectrum Analyzer with Built-in >100 GHz Preselector
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Figure 7 Block Diagram of Spurious Measurement System using
CW Signal
Figure 8 shows the measured spurious level at the input
frequency fin. The solid line in Figure 8a indicates the spu-
rious level with the preselector, while the dashed line shows
it without the preselector. Figures 8b and 8c show the
spectrum results when the input frequency fin is 115 GHz.
As shown in Figure 8b, without the preselector there are
intermodulation distortion frequency components caused by
the local signal and input signal. However, these distortion
components are not observed with the preselector (Figure
8c). Using the developed preselector supports accurate ob-
servations with no signal components that were not present
on the original input signal.
Figure 8 Spurious Level Measurement Results
3.2 Spectrum Measurement using Wideband Mod-
ulation Signal
A millimeter wave wideband modulation signal was gen-
erated for testing and measurement. Wideband modulation
signal generation used a coherent interference millimeter
wave generation method like of optical local signal genera-
tion.
Figure 9 shows the block diagram of the system for
measuring the spectrum using a wideband modulation sig-
nal. A difference frequency 4•fm optical 2-tone signal gener-
ated by the Optical LO Signal Generator was modulated
using the LN modulator and converted to a millimeter wave
signal by the UTC-PD before output9), 10). The modulation
signal source used by the LN Modulator uses an arbitrary
waveform generator (AWG). This type of configuration of-
fers an easy way to generate a wideband modulation signal
and can output a signal with both a 4•fm carrier and a 4•fm ±
double sideband from the UTC-PD.
Figure 9 Block Diagram of Spectrum Measurement System using
Wideband Modulation Signal
Figure 10 shows the measured spectrum of the wideband
millimeter wave modulation signal used for testing meas-
ured by generating a QPSK modulation wave from the AWG
with a 12.5 GHz center frequency, 10 Gsym/s symbol rate
and roll-off rate of 0.3. Figure 10a shows the measured re-
sults with the preselector removed. Without the preselector,
the lower sideband (LSB) has an image near 112.5 GHz and
distortion components due to the carrier are also observed
as spurious near 118.5 GHz. Figure 10b shows the meas-
urement results with the preselector installed in the system,
confirming that the carrier and upper sideband (USB) are
correctly observed in the measured frequency range.
These results show that using the developed preselector
in this system supports measurement without image signal
components and spurious response.
4 Conclusion
We have successfully developed a millimeter wave spec-
trum analyzer system with new built-in preselector sup-
porting measurements above 100 GHz. We have verified the
effectiveness of this system in suppressing spurious re-
sponses thereby supporting accurate measurement of CW
and wideband modulation signals.
110 120 130 140−80
−70
−60
−50
−40
−30
−20
110 120 130 140−100
−80
−60
−40
−20
0
110 120 130 140−100
−80
−60
−40
−20
0
Frequency [GHz]
Spu
riou
s L
evel P
spur [d
Bc]
w/o Pre−serectorw/ Pre−serector
Pin=−10dBm
Po
we
r [d
Bm
]
Frequency [GHz]
Po
we
r [d
Bm
]
fin=115GHz
fin=115GHz
Pspur
Pspur
Frequency [GHz]
(b)
(c)
(a)(a) Input frequency vs spurious level
(b) Spectrum (without preselector)
(c) Spectrum (with preselector)
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Anritsu Technical Review No.23 September 2015 Millimeter Wave Spectrum Analyzer with Built-in >100 GHz Preselector
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Figure 10 Wideband Modulation Signal Measurement Results
(RBW: 1 MHz, Peak Search)
Acknowledgments
Part of this research was supported by the Ministry of
Internal Affairs and Communications research project to
expand radiowave resources.
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Authors
Yukiyasi Kimura
R&D Division
R&D Center
1st Technology Development Dept.
Masaaki Fuse
R&D Division
R&D Center
1st Technology Development Dept.
Akihito Otani
R&D Division
R&D Center
1st Technology Development Dept.
and Technology Management Center
(a) Without Preselector
Carrier
USB
Image of USB
Spurious
(b) With Preselector
Carrier
USB
No Spurious
No image
Publicly available
38